IT HAS always seemed obvious to me that if you expose a person to violence at an early age then that person is likely to become violent. But of course the notion that environment could determine behavior has fallen into extreme disfavor among scientists.

After all, celebrity evolutionary theorist Steven Pinker asserts that there's a lesson to be learned when twins separated at birth have the same propensity for practical jokes in elevators. It means that behavior is innate, he claims, and (weirdly) seems to suggest that there is a genetic basis for stupid, conveyance-based merriment.

But a new study conducted in Chicagoand published last week in Sciencemakes a compelling argument against the annoying elevator twins. To track the experiences of more than 1,000 teenagers in 78 different neighborhoods, public health wonks conducting the study used a novel statistical model to demonstrate that young people exposed to gun violence are more likely to become violent themselves.

Led by Jeffrey Bingenheimer at the University of Michigan, the researchers used "propensity stratification," a method often favored by economists for generating a random sample of subjects out of what are often nonrandom circumstances (such as what the subjects' neighborhoods and families are like). As a result, Bingenheimer and his colleagues were able to correct for biases caused by things like certain kids' greater exposure to violence in high-crime areas. But even after correcting for these biases, the researchers still found that kids who were shot or shot ator who witnessed someone else being shot or shot atwere twice as likely to engage in violence later on.

Nothing like a good, solid social scientific study to remind you that environment can and does determine future behavior. Obviously, there is an ineffable combination of biological factors that can predispose someone to aggressionotherwise you'd never get homicidal people who come from "nice" backgrounds that hardly seem like the prerequisites to cruelty and murder. But where you're born and who you grow up with could really mean the difference between a life of violence and a life of peace.

There is also an argument to be made that we control our environments, and therefore if we want to change our behavior, we should change the world around usor, failing that, we should at least change our personal circumstances. That's why I'm excited about Rebuilt, a new book by science writer Michael Chorost, about what it's like to become a cyborg.

Chorost is one of thousands of deaf people all over the world who have had computers called cochlear implants surgically implanted in their skulls to restore their hearing. Providing an interface between auditory nerves in the ear and the outer world of sound, the computer uses an elaborate system of antennae and processing software to translate noise into signals that the brain learns to decode as speech, music, wind, whatever.

In Rebuilt, Chorost explores what it's like to have a computer mediating your environment and social experiences. His life is changed in all kinds of ways, not the least of which is realizing that there is no such thing as a "natural" or "true" form of sensory input. "One's most basic relationships to reality can be amended and edited and upgraded; reality is ultimately a matter of software," he writes. And we see this happening to him literally, as software upgrades to his computer alter his perceptions. When he first gets the cochlear implant, Chorost hears the characteristic "beep" of the microwave as a weird blatting noise. An upgrade restores the beeping. But what about all the kids growing up with these computers embedded in their headsand there are many of themwho don't get that particular upgrade? Are they missing something because they hear microwave ovens "wrong"? This seemingly trivial question goes to the heart of the debate over changing the world in order to change ourselves. It asks whether changing our surroundings, our physical reality, changes us as people.

Some would say that when we alter our bodies profoundly to change our social relationships, or attempt massive cultural transformations by changing educational or economic systems, that we are wasting our time. There is no way to keep Pinker's twins from being obnoxious on elevators, because it's just part of their unalterable selves. But what if we took their elevators away? Or gave them the ability to hear the sounds of discomfort their stupid jokes produced in people? Might they not consider changing their ways? With Rebuilt, Chorost seems to argue that they would. It is the most hopeful thing I've read in quite a while.